Formation of TiB2–MgAl2O4 Composites by SHS Metallurgy

TiB2–MgAl2O4 composites were fabricated by combustion synthesis involving metallothermic reduction reactions. Thermite reagents contained Al and Mg as dual reductants and TiO2 or B2O3 as the oxidant. The reactant mixtures also comprised elemental Ti and boron, as well as a small amount of Al2O3 or MgO to serve as the combustion moderator. Four reaction systems were conducted and all of them were exothermic enough to proceed in the mode of self-propagating high-temperature synthesis (SHS). The reaction based on B2O3/Al/Mg thermite and diluted with MgO was the most exothermic, while that containing TiO2/Al/Mg thermite and Al2O3 as the diluent was the least. Depending on different thermites and diluents, the combustion front temperatures in a range from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s were measured. The XRD spectra confirmed in situ formation of TiB2 and MgAl2O4. It is believed that MgAl2O4 was synthesized through a combination reaction between Al2O3 and MgO, both of which can be totally or partially produced from the metallothermic reduction of B2O3 or TiO2. The microstructure of the TiB2–MgAl2O4 composite exhibited fine TiB2 crystals surrounded by large densified MgAl2O4 grains. This study demonstrated an energy-saving and efficient route for fabricating MgAl2O4-containing composites.


Introduction
TiB 2 has been one of the most studied ultra-high temperature ceramics (UHTCs) due to its unique properties, including a high melting point (3225 • C), high hardness (33 MPa), high Young's modulus (530 MPa), excellent wear and oxidation resistance, thermal shock resistance, chemical inertness, and good electric conductivity [1][2][3]. Combination of these properties makes TiB 2 an ideal candidate for use in ballistic armors, crucibles, metal evaporation boats, cutting tools, wear resistance parts, and cathodes for alumina smelting [4][5][6]. Many ceramic phases, such as Al 2 O 3 , SiC, B 4 C, and MgAl 2 O 4 , have been considered as the reinforcement to improve fracture toughness, oxidation resistance, heat resistance, and mechanical strength of the TiB 2 -based composites [7][8][9][10]. Moreover, a recent study showed that TiB 2 -Al 2 O 3 -MgAl 2 O 4 composite possesses temperature insensitive and enhanced microwave absorption properties [11]. Magnesium aluminate spinel, MgAl 2 O 4 , as an additive has been rarely studied, possibly because the fabrication of MgAl 2 O 4 via either the direct solid-state reaction of oxides or wet chemical methods requires multiple steps that are complicated and time-consuming [12][13][14]. However, MgAl 2 O 4 is an attractive component due to its high melting point, chemical inertness, high hardness, corrosion resistance, high mechanical strength, and low cost [12].
Among various fabrication routes for preparing multiphasic ceramics, metallothermic reduction reactions (i.e., thermite reactions) combined with combustion synthesis have been recognized as a promising technique for in situ formation of MgAl 2 O 4 -containing composites [15]. Combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS), which is based on strongly exothermic reactions, has merits of low energy consumption, short reaction time, simple equipment and operation, high-purity products, and in situ formation of composite components [16][17][18]. Moreover, aluminothermic and magnesiothermic reduction reactions have Al 2 O 3 and MgO as their respective by-products, both of which are precursors for the formation of MgAl 2 O 4 . Consequently, Omran et al. [19] applied the reduction-based SHS technique to produce MgAl 2 O 4 -W-W 2 B composites through magnesiothermic reduction of B 2 O 3 and WO 3 in the presence of Al 2 O 3 . Zaki et al. [20] synthesized MoSi 2 -and Mo 5 Si 3 -MgAl 2 O 4 composites by SHS with a reducing stage from raw materials consisting of MoO 3 , SiO 2 , and Al as aluminothermic reagents and MgO as a precursor. Similarly, MgO was added into the reactive mixture of TiO 2 , B 2 O 3 , and Al to fabricate TiB 2 -MgAl 2 O 4 composites by thermitic combustion synthesis [21]. Generally, most of the previous studies on the formation of MgAl 2 O 4 -containing composites had prior addition of one of two precursors (Al 2 O 3 or MgO) in the green samples.
This study represents the first attempt to prepare TiB 2 -MgAl 2 O 4 composites from the SHS powder metallurgy simultaneously involving aluminothermic and magnesiothermic reduction of TiO 2 or B 2 O 3 . Only a small amount of Al 2 O 3 or MgO was included in the reactive mixture to serve as the combustion moderator and part of the precursors for the formation of MgAl 2 O 4 . Four SHS reaction systems formulated with different metallothermic reagents and combustion diluents were investigated. In this work, combustion exothermicity and kinetics of the combustion wave of the SHS process, as well as compositions and microstructures of the final products were explored.
Combustion exothermicity of the above four reactions, Equations (1)-(4), was evaluated by calculating their adiabatic combustion temperatures (T ad ) from the following energy balance equation [17,22] with thermochemical data taken from [23].
where ∆H r is the reaction enthalpy at 298 K, c p and L are the heat capacity and latent heat, n j is the stoichiometric coefficient, and P j refers to the product component.
The SHS experiments were conducted in a windowed combustion chamber filled with Ar at 0.25 MPa. Reactant powders were well mixed in a tubular ball mill and then uniaxially compressed into cylindrical test specimens with a diameter of 7 mm, a height of 12 mm, and a relative density of 55%. The sample compact was ignited by an electrically heated tungsten coil. An R-type bare-wire thermocouple (Pt/Pt-13%Rh) with a bead size of 125 µm was used to measure the combustion temperature. The propagation velocity of combustion wave (V f ) was determined by the time derivative of the flame-front trajectory constructed from the recorded series of combustion images. Phase compositions of the products were identified by an X-ray diffractometer (XRD, Bruker D2 Phaser, Karlsruhe, Germany). Microstructures and constituent elements of the products were examined by the scanning electron microscopy (SEM, Hitachi, Tokyo, Japan, S3000H) and energy dispersive spectroscopy (EDS). Details of the experimental methods were reported elsewhere [24]. Figure 1 presents the calculated ∆H r and T ad of reactions Equations (1)-(4) and shows that Equation (4) has the highest values while Equation (1) has the lowest ones. Both ∆H r and T ad increase from Equations (1)- (4). Specifically, the values of T ad are 2530 K, 2595 K, 2783 K, 2897 K for Equation (1)-(4), respectively. A comparison between Equations (1) and (2) revealed that the combustion moderator Al 2 O 3 appeared to impose a stronger dilution effect on combustion than MgO, which led to a lower T ad for Equation (1) than Equation (2). Similar results were observed in Equations (3) and (4). These findings could also be explained by the fact that metallothemic reduction of TiO 2 or B 2 O 3 by Al is more exothermic than that by Mg [15,25].

Combustion Exothermicity of Reduction-Based SHS Reactions
cally heated tungsten coil. An R-type bare-wire thermocouple (Pt/Pt-13%Rh) with a bead size of 125 μm was used to measure the combustion temperature. The propagation velocity of combustion wave (Vf) was determined by the time derivative of the flame-front trajectory constructed from the recorded series of combustion images. Phase compositions of the products were identified by an X-ray diffractometer (XRD, Bruker D2 Phaser, Karlsruhe, Germany). Microstructures and constituent elements of the products were examined by the scanning electron microscopy (SEM, Hitachi, Tokyo, Japan, S3000H) and energy dispersive spectroscopy (EDS). Details of the experimental methods were reported elsewhere [24].  (1) and (2) revealed that the combustion moderator Al2O3 appeared to impose a stronger dilution effect on combustion than MgO, which led to a lower Tad for Equation (1) than Equation (2). Similar results were observed in Equations (3) and (4). These findings could also be explained by the fact that metallothemic reduction of TiO2 or B2O3 by Al is more exothermic than that by Mg [15,25].

Combustion Exothermicity of Reduction-Based SHS Reactions
On the other hand, because the B2O3-based thermite is more energetic than the one using TiO2 [25], Equation (3) has a higher Tad than Equation (1). Similarly, Equation (4) has a higher Tad than Equation (2). According to the calculated Tad, it is realized that the thermite oxidants (i.e., B2O3 versus TiO2) have a more pronounced influence on combustion exothermicity than the diluent oxides (i.e., Al2O3 versus MgO).

Combustion Synthesis Reactions
Enthaply of Reaction,

Combustion Temperature and Self-Propagating Velocity
Two series of the SHS processes recorded from reactions Equations (1) and (3) are illustrated in Figure 2a,b, respectively. It is apparent that upon ignition, the reaction was On the other hand, because the B 2 O 3 -based thermite is more energetic than the one using TiO 2 [25], Equation (3) has a higher T ad than Equation (1). Similarly, Equation (4) has a higher T ad than Equation (2). According to the calculated T ad , it is realized that the thermite oxidants (i.e., B 2 O 3 versus TiO 2 ) have a more pronounced influence on combustion exothermicity than the diluent oxides (i.e., Al 2 O 3 versus MgO).

Combustion Temperature and Self-Propagating Velocity
Two series of the SHS processes recorded from reactions Equations (1) and (3) are illustrated in Figure 2a,b, respectively. It is apparent that upon ignition, the reaction was initiated and characterized by a self-sustaining combustion wave. More intense combustion accompanied with a faster combustion wave was observed in Figure 2b, when compared with that in Figure 2a. Combustion luminosity and flame spreading speed reflected the degree of reaction exothermicity. As mentioned above, B 2 O 3 /Al/Mg-based Equation (3) is more energetic than TiO 2 /Al/Mg-based Equation (1). Similar combustion behavior was also noticed in Equations (2) and (4). reaction zone. The peak value is considered as the combustion front temperature (Tc). When compared with pinnacles in the contours of Equations (1) and (2), sharper peaks were detected in the profiles of Equations (3) and (4). This implied a faster combustion wave in Equations (3) and (4). As shown in Figure 3, the values of Tc from Equation (1) to Equation (4) in ascending order are 1348 °C, 1445 °C, 1660 °C, and 1736 °C. It should be noted that the measured combustion front temperatures are in agreement with the calculated reaction exothermicity.  (1) and (b) Equation (3). Figure 3 depicts typical combustion temperature profiles measured from four different reactions. All profiles exhibit a steep temperature rise followed by a rapid descent, which is characteristic of the SHS reaction that features a fast combustion wave and a thin reaction zone. The peak value is considered as the combustion front temperature (T c ). When compared with pinnacles in the contours of Equations (1) and (2), sharper peaks were detected in the profiles of Equations (3) and (4). This implied a faster combustion wave in Equations (3) and (4). As shown in Figure 3, the values of T c from Equation (1)-(4) in ascending order are 1348 • C, 1445 • C, 1660 • C, and 1736 • C. It should be noted that the measured combustion front temperatures are in agreement with the calculated reaction exothermicity. initiated and characterized by a self-sustaining combustion wave. More intense combustion accompanied with a faster combustion wave was observed in Figure 2b, when compared with that in Figure 2a. Combustion luminosity and flame spreading speed reflected the degree of reaction exothermicity. As mentioned above, B2O3/Al/Mg-based Equation (3) is more energetic than TiO2/Al/Mg-based Equation (1). Similar combustion behavior was also noticed in Equations (2) and (4). Figure 3 depicts typical combustion temperature profiles measured from four different reactions. All profiles exhibit a steep temperature rise followed by a rapid descent, which is characteristic of the SHS reaction that features a fast combustion wave and a thin reaction zone. The peak value is considered as the combustion front temperature (Tc). When compared with pinnacles in the contours of Equations (1) and (2), sharper peaks were detected in the profiles of Equations (3) and (4). This implied a faster combustion wave in Equations (3) and (4). As shown in Figure 3, the values of Tc from Equation (1) to Equation (4)   It is useful to note in Figure 3 that the curves of Equations (3) and (4) have a shape peak with a pronounced shoulder. The shape peak was a result of the fast combustion wave. The pronounced shoulder could be caused by the occurrence of volumetric synthesis reactions after the passage of the rapid combustion wave.  (1)-(4) is consistent with that of T c . This can be understood by the fact that the propagation of combustion wave is essentially governed by layer-by-layer heat transfer from the reaction zone to unreacted region, and therefore, is subject to the combustion front temperature. As presented in Figure 4, the average combustion velocities are 3.9, 4.7, 5.1, and 5.7 mm/s for Equations (1)-(4), respectively. It is worth noting that the measured combustion temperature not only justified the reaction exothermic analysis, but confirmed the temperature dependence of combustion wave velocity.
It is useful to note in Figure 3 that the curves of Equations (3) and (4) have a shape peak with a pronounced shoulder. The shape peak was a result of the fast combustion wave. The pronounced shoulder could be caused by the occurrence of volumetric synthesis reactions after the passage of the rapid combustion wave. Figure 4 plots the measured combustion wave propagation velocities (Vf) and temperatures (Tc) of four reactions. The rising trend of Vf from Equations (1) to (4) is consistent with that of Tc. This can be understood by the fact that the propagation of combustion wave is essentially governed by layer-by-layer heat transfer from the reaction zone to unreacted region, and therefore, is subject to the combustion front temperature. As presented in Figure 4, the average combustion velocities are 3.9, 4.7, 5.1, and 5.7 mm/s for Equation (1), Equation (2), Equation (3), and Equation (4), respectively. It is worth noting that the measured combustion temperature not only justified the reaction exothermic analysis, but confirmed the temperature dependence of combustion wave velocity.

Composition and Microstructure Analyses of Synthesized Products
The XRD spectra of the final products synthesized from Equations (1) and (2) are shown in Figure 5a,b, respectively. Both indicated the formation of TiB2 and MgAl2O4 along with two minor phases, magnesium titanate (MgTiO3) and MgO. It is believed that MgAl2O4 was synthesized through a combination reaction between Al2O3 and MgO. Equation (1) and (2) were formulated with the same thermite reagents of TiO2, Al, and Mg, but diluted by different metal oxides. That is, Al2O3 was partly pre-added and partly thermite-produced, while MgO was completely generated from the reduction of TiO2 by Mg in Equation (1). In contrast, the required Al2O3 in Equation (2) was entirely produced from the reduction of TiO2 by Al, but MgO was supplied in part from prior addition and in part from the reduction of TiO2 by Mg. For both Equations (1) and (2), TiB2 was synthesized from the reaction of elemental boron with reduced and metallic Ti.
Traces of MgO suggested an incomplete reaction due probably to the relatively low reaction temperatures of Equations (1) and (2). The presence of MgTiO3 in the final products of Equations (1) and (2) could be attributed to the reaction of MgO with the thermite oxidant TiO2 [26,27]. The formation of MgTiO3 in the SHS-produced TiB2-MgAl2O4 composites was also observed by Radishevskaya et al. [10] using Ti, boron, and

Composition and Microstructure Analyses of Synthesized Products
The XRD spectra of the final products synthesized from Equations (1) and (2) are shown in Figure 5a,b, respectively. Both indicated the formation of TiB 2 and MgAl 2 O 4 along with two minor phases, magnesium titanate (MgTiO 3 ) and MgO. It is believed that MgAl 2 O 4 was synthesized through a combination reaction between Al 2 O 3 and MgO. Equation (1) and (2) were formulated with the same thermite reagents of TiO 2 , Al, and Mg, but diluted by different metal oxides. That is, Al 2 O 3 was partly pre-added and partly thermite-produced, while MgO was completely generated from the reduction of TiO 2 by Mg in Equation (1). In contrast, the required Al 2 O 3 in Equation (2) was entirely produced from the reduction of TiO 2 by Al, but MgO was supplied in part from prior addition and in part from the reduction of TiO 2 by Mg. For both Equations (1) and (2), TiB 2 was synthesized from the reaction of elemental boron with reduced and metallic Ti.
Traces of MgO suggested an incomplete reaction due probably to the relatively low reaction temperatures of Equations (1) and (2). The presence of MgTiO 3 in the final products of Equations (1) and (2) could be attributed to the reaction of MgO with the thermite oxidant TiO 2 [26,27]. The formation of MgTiO 3 in the SHS-produced TiB 2 -MgAl 2 O 4 composites was also observed by Radishevskaya et al. [10] using Ti, boron, and MgAl 2 O 4 as their starting materials and a partial decomposition of MgAl 2 O 4 during combustion synthesis was considered as a possible route resulting in the formation of MgTiO 3 .
The presence of MgO along with no detection of Al 2 O 3 in the final products of Equations (1) and (2) suggested that the as-synthesized MgAl 2 O 4 is an Al 2 O 3 -rich spinel. The formation of MgTiO 3 could also result in the production of Al 2 O 3 -rich spinel. According to Naghizadeh et al. [28], magnesium titanate compounds (MgTiO 3  MgTiO3 ceramic has been proved to be an excellent dielectric material, owing to its high dielectric constant, low dielectric loss, high value of quality factors, and good temperature stability [29,30]. It is believed that a trace amount of MgTiO3 as a minor phase existed in the as-synthesized TiB2-MgAl2O4 products has no effect on the refractory properties of the composites. However, removal of MgTiO3 from the TiB2-MgAl2O4 composite would be difficult, since it could combine with MgAl2O4 in a solid solution form [28].   The SEM image shown in Figure 7 illustrates the microstructure of fracture surface of the product synthesized from Equation (1) which contains a TiO2/Al/Mg-based thermite. The morphology displays several large and solidified MgAl2O4 aggregates of 5-15 μm surrounded by fine-grain TiB2 crystals with a particle size of about 1-2 μm. Moreover, EDS analysis of two characteristic regions in the product surface indicates that the MgTiO 3 ceramic has been proved to be an excellent dielectric material, owing to its high dielectric constant, low dielectric loss, high value of quality factors, and good temperature stability [29,30]. It is believed that a trace amount of MgTiO 3 as a minor phase existed in the as-synthesized TiB 2 -MgAl 2 O 4 products has no effect on the refractory properties of the composites. However, removal of MgTiO 3 from the TiB 2 -MgAl 2 O 4 composite would be difficult, since it could combine with MgAl 2 O 4 in a solid solution form [28].
The SEM image shown in Figure 7 illustrates the microstructure of fracture surface of the product synthesized from Equation (1)   The SEM image shown in Figure 7 illustrates the microstructure of fracture surface of the product synthesized from Equation (1)  For the final product of B2O3/Al/Mg-based Equation (4), the microstructure and elemental ratios of the components are presented in Figure 8. As can be seen, MgAl2O4 was formed as large densified aggregates of around 20 μm and TiB2 crystals were in a short-rod form with a length of 2-4 μm or in a shape of fine grains of 1-2 μm.  In summary, the addition of MgAl 2 O 4 into TiB 2 enhanced the refractory properties, such as the high-temperature oxidation and corrosion resistance and thermal shock resistance [10,31,32]. Like many sintering aids, MgAl 2 O 4 as an additive could improve densification of TiB 2 ceramics and reduce sintering temperatures [33,34]. The abnormal grain growth could be efficiently prevented during the sintering process. As a result, it is more likely to obtain a uniform grain distribution.
Moreover, the TiB 2 -MgAl 2 O 4 composite is a promising high-temperature microwave absorption material with a reflection loss less than -5 dB at 8.2-18.0 GHz in the temperature range of 25 • C to 1100 • C [11]. The composite also exhibits an extremely high tolerance against intense irradiation in harsh environments [35,36]. Therefore, the potential uses of the TiB 2 -MgAl 2 O 4 composite might include heat-resistant coatings, nozzles and nose cones of supersonic jets, microwave absorption components, diagnostic or detector windows in fusion devices, target materials in the nuclear applications, etc., [11,35,36].   In summary, the addition of MgAl2O4 into TiB2 enhanced the refractory properties, such as the high-temperature oxidation and corrosion resistance and thermal shock resistance [10,31,32]. Like many sintering aids, MgAl2O4 as an additive could improve densification of TiB2 ceramics and reduce sintering temperatures [33,34]. The abnormal grain growth could be efficiently prevented during the sintering process. As a result, it is more likely to obtain a uniform grain distribution.
Moreover, the TiB2-MgAl2O4 composite is a promising high-temperature microwave absorption material with a reflection loss less than -5 dB at 8.2-18.0 GHz in the temperature range of 25 °C to 1100 °C [11]. The composite also exhibits an extremely high tolerance against intense irradiation in harsh environments [35,36]. Therefore, the potential uses of the TiB2-MgAl2O4 composite might include heat-resistant coatings, nozzles and nose cones of supersonic jets, microwave absorption components, diagnostic or detector windows in fusion devices, target materials in the nuclear applications, etc., [11,35,36].

Conclusions
In situ formation of 3TiB2-MgAl2O4 composites was conducted by combustion synthesis combined with metallothermic reduction reactions involving Al and Mg as dual reductants. Thermite reagents with different oxidants were considered; one utilized TiO2 and the other B2O3. The reactant mixtures also contained elemental Ti and boron. This study in total completed four SHS reactions, within which a small amount of Al2O3 or MgO was included in the reactive mixture to serve as the combustion moderator and part of the precursors for the formation of MgAl2O4. The overall synthesis reaction was exothermic enough to proceed in the SHS mode. An energy-saving and efficient fabrication route for the formation MgAl2O4-containing composites was demonstrated.
The analysis of combustion exothermicity indicated that the SHS reaction containing B2O3/Al/Mg-based thermites was more energetic than that adopting TiO2 as the oxidant. Prior addition of Al2O3 had a greater cooling effect on combustion than that of MgO. Depending on different thermites and diluents, the measured combustion front temperatures ranged from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s. The temperature dependence of combustion wave velocity was justified. The XRD analysis confirmed in situ formation of TiB2 and MgAl2O4. A small amount of MgTiO3 was found as the impurity. It is believed that MgAl2O4 was synthesized through a combination reaction between Al2O3 and MgO, both of which can be totally or partially 5 m

Conclusions
In situ formation of 3TiB 2 -MgAl 2 O 4 composites was conducted by combustion synthesis combined with metallothermic reduction reactions involving Al and Mg as dual reductants. Thermite reagents with different oxidants were considered; one utilized TiO 2 and the other B 2 O 3 . The reactant mixtures also contained elemental Ti and boron. This study in total completed four SHS reactions, within which a small amount of Al 2 O 3 or MgO was included in the reactive mixture to serve as the combustion moderator and part of the precursors for the formation of MgAl 2 O 4 . The overall synthesis reaction was exothermic enough to proceed in the SHS mode. An energy-saving and efficient fabrication route for the formation MgAl 2 O 4 -containing composites was demonstrated.
The analysis of combustion exothermicity indicated that the SHS reaction containing B 2 O 3 /Al/Mg-based thermites was more energetic than that adopting TiO 2 as the oxidant. Prior addition of Al 2 O 3 had a greater cooling effect on combustion than that of MgO. Depending on different thermites and diluents, the measured combustion front temperatures ranged from 1320 to 1720 • C, and combustion wave velocity from 3.9 to 5.7 mm/s. The temperature dependence of combustion wave velocity was justified. The XRD analysis confirmed in situ formation of TiB 2 and MgAl 2 O 4 . A small amount of MgTiO 3 was found as the impurity. It is believed that MgAl 2 O 4 was synthesized through a combination reaction between Al 2 O 3 and MgO, both of which can be totally or partially produced from the metallothermic reduction of B 2 O 3 or TiO 2 . The microstructure of the synthesized composite exhibited that MgAl 2 O 4 was surrounded by closely packed TiB 2 grains. MgAl 2 O 4 was formed as densified aggregates with a size of 5-20 µm. TiB 2 crystals were produced in a shape of short rods of 2-4 µm and fine grains of 1-2 µm.